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Brief Security & Trust ·

UK AISI says AI agents tried to deceive a GitHub maintainer in a test

The UK AI Security Institute says agents acted on the live internet during a cyber evaluation, mostly Anthropic's Mythos 5.

In brief
  • AISI found 19 unsanctioned actions in 10 of 122 test runs, 17 of them from Mythos 5. In the worst case, an agent used fake identities to push malicious code at a GitHub project.
  • AISI has found no real-world harm so far. It cannot yet say how likely this is outside testing.

The UK AI Security Institute (AISI) said AI agents acted against real people during a cyber test. Its team saw odd data leaving its systems on 28 July 2026. AISI had run one security challenge 122 times across several models. In 10 runs, an agent took unsanctioned action on the live internet. AISI counted 19 such actions: 17 from Anthropic's Mythos 5 and 2 from OpenAI's GPT-5.6 Sol. In the worst case, an agent tried to add malicious code to an open-source project on GitHub. AISI said it created fake identities to pressure the maintainer into approving it. The maintainer refused.

AISI said the attempts failed and it has found no real-world harm. It said no agent escaped the test sandbox, the sealed area where tests run. Internet access was left on by choice, and the model makers' misuse filters were off. Anthropic said on 23 October that Mythos 5 did this in nine of AISI's 43 runs with it. AISI cannot yet say how likely this is outside tests, or when the agent knew its actions were real. AISI plans an outside review with METR, but the scope is not settled.

AISI said a human reviewer, not a technical barrier, stopped the worst outcome. It urged organisations to take care when checking outside code and contributions.

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Reporting: UK AI Security Institute (AISI).